This disclosure relates to turbines and, more particularly, to extending the flow range of turbines.
Power generation turbines and alternators are used for downhole drilling operations to supply electrical power to electronic components used for measuring, logging, or sampling while drilling. As drilling mud passes through a stationary blade row in the turbine, it generates an angular momentum, or flow swirl, in expense of the pressure differential. The downstream rotating blade row, or rotor, converts that angular momentum, as well as its own reaction, into the shaft power, and supplies it to an alternator to generate electricity. During operation, the power generation turbine has to operate within a range of flow rates and as dictated by job operating conditions. This limited range of turbine operation typically does not cover the entire rig operating flow rate range that can be expected for a particular tool size. A turbine operating below an optimal flow rate range may produce insufficient power for the electronic components. A turbine operating above an optimal flow rate range may experience relatively high thermal stresses and/or accelerated wear of attached mechanical components, thus reducing reliability and service life. Moreover, replacing a damaged or worn turbine may be time-consuming and expensive and, in some instances, may be impossible once the turbine is installed downhole. Moreover, an operator may mistakenly select a turbine that is not optimized for the particular flow rate.
Additionally, the mud flow in the turbine typically contains suspended solid particles, such as sand. These particles, passing at a high speed across the turbine blade rows and especially at conditions outside of the turbine's flow rate range, can cause erosion to the blades or downstream turbine components. The replacement of these eroding parts may increase overall maintenance material and supply (M&S) tool costs and increase service frequency.